Gas turbine engines conventionally include several fuel nozzles to spray fuel into the combustors. Generally the nozzles are mounted to a wall of the engine housing and are spaced circumferentially apart around the periphery of the combustor to dispense the fuel in a generally circumferential pattern.
Since the fuel nozzles and their support stems are located in a very hot area of the engine, the fuel passing through the support stem can rise to a temperature sufficient to decompose or carbonise the fuel resulting in coking that can clog the nozzles and prevent the nozzles from spraying fuel properly into the combustor. Therefore, fuel nozzle support stems are generally constructed with some form of insulation and fuel cooling system to prevent the fuel from heating to a temperature, which would produce coke. In operating conditions where nozzle stems conduct a low volume of fuel or fuel flow becomes stagnant, the fuel can become heated during a long residence time in the hot combustor environment. An insulating sleeve of sheet metal is generally used to provide an insulating air gap that partially shields the nozzle stem from excess heating. Various methods are also conventionally used to circulate relatively cool fuel through the nozzle support stem in order to provide a flow of cooling liquid to regulate the temperature of the stem and control heat transfer to fuel flowing through the stem.
A common cooling system utilised for dual fuel nozzles is where a primary fuel tube and a secondary fuel tube are concentrically disposed within the support stem so as to define two distinct conduits for directing primary and secondary fuel flows. The primary fuel is conveyed through a conduit of circular cross section defined by the primary fuel tube while the secondary fuel is delivered through the annular space defined between the primary fuel tube and the secondary fuel tube.
U.S. Pat. No. 4,735,044 to Richie et al. shows a dual fuel path stem with an inner primary fuel tube and outer tube housed within a hollow tubular stem. All three components are concentric and bent to a desired configuration during manufacture. A distinct disadvantage of the bent tube stem is that accurate positioning of the nozzles becomes extremely difficult. The tubes tend to straighten or deform in a heated environment such that the nozzle tip is displaced as a result of thermal expansion. This thermally induced movement is substantially worsened where the structure is asymmetric or where the cooling is unbalanced. Such displacements in the location of the nozzle tip can significantly effect performance, emissions and reliability of the combustion system.
A further example of concentric tube construction is shown in U.S. Pat. No. 5,577,386 to Alary et al. The manufacture of such concentric tubes to accurate dimensions is very difficult. In addition, due to the complex shape of the stem structure and use of thin walled tubes, the thermal deformation in a heated environment is difficult to predict. As a result of the combined inaccuracy from manufacture and heat distortion, the precise location and orientation of the fuel nozzles is below the standards required for aircraft engine manufacture. Tolerances for the position and orientation of fuel nozzles are becoming very stringent as a result of a drive to improve the fuel efficiency, to reduce environmental pollution in operation of gas turbine engines, and to ensure that the flow requirements to the turbine sections of the engine are maintained.
Fuel nozzles and their support stems are conventionally constructed of thin wall tubes in order to minimise aircraft engine weight. However, as mentioned above, the bending of concentric thin wall tubes during manufacture is a difficult procedure to perform accurately especially where extremely close tolerances are essential. In addition, thin wall tubes even if equipped with insulating air gap sleeves or circulating fuel-cooling systems nevertheless experience significant distortion in an extremely hostile high temperature-high turbulence environment. This in turn results in deterioration in combustor performance and increased exhaust emissions.
On the other hand, since nozzles must be frequently serviced and the fuel conduits within the support stems often accumulate coke, these engine components are frequently replaced during overhaul of an engine. As a result any significant increase in the cost of manufacturing nozzles and fuel stems to more precise tolerances multiplies the increase in the cost of overhauls and engine maintenance.
It is an object of the invention to provide a nozzle support stem structure which can be easily machined and assembled to extremely close manufacturing tolerances with improved structural strength and dynamic stability to improve engine fuel efficiency, emissions and operability through accurate orientation and location of fuel nozzles.
It is an object of the present invention to provide a low cost easily manufactured and maintained nozzle support stem which incorporates a dual fuel cooling system to prevent fuel coking and to minimise the effects of heat distortion.
It is a further object of the invention to provide a nozzle support stem structure assembled from modular components to reduce manufacturing costs, and engine overhaul costs.
Further objects of the invention will be apparent from review of the drawings and description of the invention below.